1. Field of the Invention
The present invention relates to a layered high frequency filter incorporating at least three resonators.
2. Description of the Related Art
With increasing demands for reductions in dimensions and thickness of communications apparatuses conforming to the Bluetooth standard and those for use on a wireless local area network (LAN), techniques for high-density packaging has been required. One of proposals for meeting such a requirement is to integrate components through the use of a layered substrate.
One of components of the above-mentioned communications apparatuses is band-pass filters that filter reception signals. As the band-pass filters, layered band-pass filters such as those disclosed in JP 2003-318605A and JP 2001-053502A are known. The layered band-pass filters each incorporate a plurality of resonators formed using conductor layers of a layered substrate. In each of the layered band-pass filters, respective adjacent ones of the resonators are inductively coupled to each other. For the layered band-pass filters, as disclosed in the above-mentioned publications, there are cases in which the respective adjacent ones of the resonators are also capacitively coupled to each other. In such cases, it is possible to adjust the frequencies of two attenuation poles and the pass-band width of each of the band-pass filters by adjusting the magnitude of the inductive coupling and the magnitude of the capacitive coupling. Thus, capacitively coupling the respective adjacent ones of the resonators to each other makes it easier to adjust the characteristics of the band-pass filters, compared with a case in which the respective adjacent ones of the resonators are not capacitively coupled to each other.
JP 2001-053502A discloses a technique of capacitively coupling non-adjacent resonators to each other through the use of a series circuit including a transmission line and two bypass capacitors. The filter disclosed in this publication incorporates: three resonator electrodes; two inter-stage coupling capacitor electrodes that are respectively opposed to two of the resonator electrodes adjacent to each other; and one bypass electrode opposed to the two inter-stage coupling capacitor electrodes. In this filter, two inter-stage coupling capacitors that capacitively couple two adjacent resonators to each other are formed of the adjacent two of the resonator electrodes and one of the inter-stage coupling capacitor electrodes opposed to the two of the resonator electrodes. In addition, two bypass capacitors are formed of the two inter-stage coupling capacitor electrodes and the bypass electrode opposed to the two inter-stage coupling capacitor electrodes. It is possible, through this technique, to control the attenuation pole by adjusting the capacitance between the non-adjacent resonators without being affected by the magnetic field coupling between the non-adjacent resonators.
In the technique disclosed in JP 2001-053502A, the inter-stage coupling capacitor electrodes are shared between the inter-stage coupling capacitors and the bypass capacitors. As a result, according to this technique, it is difficult to control the capacitance of the bypass capacitors independently of the capacitance of the inter-stage coupling capacitors. In addition, it is impossible through this technique to capacitively couple the non-adjacent resonators to each other without capacitively coupling adjacent resonators to each other. It is therefore difficult to adjust the characteristics of the band-pass filter through this technique.